An ignition coil is provided to an internal combustion engine, such as a vehicular engine. The ignition coil includes a primary coil and secondary coil arranged coaxially with each other. The ignition coil generates high voltage of 20 to 40 kV in the secondary coil by utilizing electromagnetic induction. The secondary coil is connected with a sparkplug including a pair of electrodes. The electrodes spark by being applied with the high voltage, thereby causing combustion in the internal combustion engine. Ignition coils include various kinds of insulating resin such as polyphenylene sulfide (PPS), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polyphenylene ether (PPE), and epoxy resin being resistive against high voltage.
For example, JP-A-8-339928 discloses an ignition coil for internal combustion engine. In the ignition coil of JP-A-8-339928, a case of the ignition coil and a primary bobbin are formed of PPS resin instead of conventional PBT resin. In this structure, the ignition coil is downsized while maintaining output energy.
In recent years, development of vehicular engines has been proceeded to achieve high output, low fuel consumption, and low emission. In addition, an ignition system is demanded to produce higher voltage year by year. Therefore, it is demanded to improve an insulated member for an ignition coil to enhance corona lifetime corresponding to durability when being applied with high voltage for a long period.